Thick Plasma Research Articles

Enablement or Suppression of Collisionless Magnetic Reconnection by the Background Plasma Beta and Guide Field.

How magnetic reconnection is triggered or suppressed is an important outstanding problem. By considering pinching of a current sheet that has formed at non-equilibrium, we show that the background plasma beta is a major controlling factor in the onset and nature of magnetic reconnection. A high plasma beta inhibits a current sheet from pinching down to kinetic scales required for collisionless reconnection, while a low beta facilitates it. A simple adiabatic model provides a good prediction for the reconnection-enabled regions in thickness versus peak plasma beta space, which are confirmed by a series of particle-in-cell simulations with varying initial parameters. A strong dependency of the peak reconnection rate on the plasma beta is clearly predicted with reconnection being favored in low beta conditions. A finite guide field is an additional source of reconnection suppression, consistent with previous observations that reconnection requires a large enough magnetic shear angle for high-beta situations.

Modeling the electron cyclotron emission radiation signature from suprathermal electrons in a tokamak.

An Electron Cyclotron Emission (ECE) modeling code has been developed to model ECE radiation with an arbitrary electron momentum distribution, a small oblique angle, both ordinary (O-mode) and extraordinary polarizations (X-mode), and multiple cyclotron frequency harmonics. The emission and absorption coefficients are calculated using the Poynting theorem from the cold plasma dispersion and the electron-microwave interaction from the full anti-Hermitian tensor. The modeling shows several ECE radiation signatures that can be used to diagnose the population of suprathermal electrons in a tokamak. First, in an n = 2 X-mode (X2) optically thick plasma and oblique ECE view, the modeling shows that only suprathermal electrons, which reside in a finite region of the velocity and space domains, can effectively generate cyclotron emissions to the ECE receiver. The code also finds that the O1 mode is sensitive to suprathermal electrons of both a high v⊥ and v‖, while the X2 mode is dominantly sensitive to suprathermal electrons of a high v⊥. The modeling shows that an oblique ECE system with both X/O polarization and a broad frequency coverage can be used to effectively yield information of the suprathermal electron population in a tokamak.

Hydrogen recombination continua in stellar flares

ABSTRACT An increasing interest in stellar flares stimulated various modelling approaches in order to analyse the observed flare fluxes. A particular interest was focused on photometric data obtained from Kepler and TESS satellites which detected thousands of flares on cool dwarf stars, including extremely energetic superflares. Radiation-hydrodynamical simulations, together with a rather rare broad-band spectroscopy, indicate much larger densities in the superflare chromospheres as compared to solar flares. Formation of hydrogen recombination continua under such different densities ranging from 1013 to 1015 cm−3 or more is governed by physics of optically thin to largely thick plasmas, the continuum optical thickness being within the range of four orders of magnitude. Various authors presented simple approximate methods to analyse the photometric data from Kepler or TESS under such diverse regimes of physical conditions. In this letter, we summarize the general physical approach and compute the hydrogen recombination spectra under the above range of electron densities. We show the theoretical contrast with respect to quiet-star continuum for two characteristic stars of G and dMe type. Based on that we distinguish three regimes of the continuum formation and discuss the applicability of various simple approaches.

Optical characterization and dispersion analyses of plasma polymerized methyl acrylate thin films

This work reports the structural characteristics, surface morphology, linear and nonlinear optical properties of 110 to 225 nm thick plasma polymerized methyl acrylate (PPMA) thin films. X-ray diffraction analyses confirm the amorphous nature of the films. Field emission scanning electron micrographs of the films display cluster-based surface morphology. Attenuated total reflectance Fourier transform infrared spectroscopy confirms the chemical structural changes in the films. The optical properties were studied based on the absorbance, transmittance, and reflectance spectra measured by an ultraviolet–visible spectrophotometer within the wavelength ranges from 200 to 800 nm. The direct optical band gap and Urbach values are increased from 3.66 to 3.83 eV and 0.28 to 0.45 eV, respectively with increasing film thickness. The extinction coefficient and refractive index were evaluated, and discussed a correlation between the refractive index and the optical bandgap. The real and imaginary dielectric constants, volume/surface energy loss functions and skin depth were deduced. The oscillator energies and parameters were analyzed using the concept of Wemple-DiDomenico and Sellmeier models, respectively for a single oscillator. Static linear refractive index for the studied films exhibits normal dispersion behavior with film thicknesses and satisfied Moss, Ravindra-Gupta, and Herve-Vandamme rules. The linear susceptibility, third-order nonlinear susceptibility and the non-linear refractive index are considerably reduced from 0.20, 29.5 × 10−14 esu, and 5.89 × 10−12 esu with increasing optical band gap energies. The outcomes from the analyses of PPMA demonstrated their potential for usage in electronic, optoelectronic, and non-linear device applications.

Self-absorption correction method based on intensity self-calibration of doublet lines.

The self-absorption effect in an optically thick plasma seriously affects the spectral line intensity and the measurement accuracy of laser-induced breakdown spectroscopy (LIBS). In this work, a self-absorption correction method based on intensity self-calibration of doublet lines belonging to the same multiplet is proposed. The K/Δλ0 parameter and self-absorption coefficient (SA) of the doublet lines of the analytical element can be calculated based on the measured actual lines intensity ratio and the K parameters ratio. Compared with the generally applied self-absorption correction methods, this method can effectively reduce the influence of laser energy and plasma plume fluctuations and the non-uniformity distribution of the element in the plasma, and is independent of the availability of Stark broadening coefficients. So it has obvious advantages of high computation efficiency, high analysis accuracy and good applicability. Univariate quantitative analysis results of aluminum (Al) show that the correlation coefficient of calibration curves and the measurement accuracy of elemental content have been significantly improved with the self-absorption correction.

120 mA operation of J-PARC cesiated RF-driven H- ion source

The Japan Proton Accelerator Research Complex (J-PARC) cesiated RF-driven H- ion source (IS) has been stably operated for about eight years by using an internal-RF-antenna developed at the Spallation Neutron Source (SNS). From Sep. 2018, the J-PARC 400 MeV LINAC has been steadily operated with the design H- ion beam intensity (IH- ) of 50 mA, when an about 60 mA beam is injected from the IS. At each end of about 3 months J-PARC operation, the IS supplies a 52.5 keV 72 mA beam for the LINAC 60 mA acceleration. The IS superior emittances predicted in a test-stand were proven by the high RFQ acceleration efficiency of 94.3% (IH-RFQ = 67.9 mA). The high intensity beam with transverse emittances suitable for the RFQ is produced with several unique measures, such as, slight water molecules addition into hydrogen plasma, the low temperature (about 70°C) operation of 45°-tapered 16-mm thick plasma electrode with the precise cesium (Cs) density control, the impurity elimination in the hydrogen plasma along with the filter-field optimization, the continuous-wave igniter plasma driven with a 42-W 30-MHz RF, and so on. The 120 mA operation results of the IS in the test-stand are presented in this paper. A 69.9 keV 120 mA H- ion beam, about 107.8 mA of which was inside the transverse emittances used for a common RFQ design, was stably operated with a duty factor of 4% (1 ms × 40 Hz). The IS is the benchmark one for the next generation high energy H- LINACs aiming for 100 mA class intensity.

Offset bending for interfacial toughness of plasma sprayed ceramic coatings

A modified three-point bending technique has been proposed to measure the interfacial toughness of thick plasma sprayed ceramic coatings. Four different ceramic materials (titania, yttria stabilized zirconia, alumina, and yttria) on steel substrate were investigated resulting in a range of toughness values. Plasma sprayed coatings were prepared on grit blasted substrates with two different roughnesses and subsequently a notch in the centre of the coated beam was fabricated using a SiC saw. A two-step three-point bend test was conducted: first, the notched sample was loaded with a small offset to create pre-crack. The pre-cracked specimens were further loaded with offset 0.5 to measure interfacial toughness. Crack propagation manifested as a pop-in in the load-deflection plot. Subsequently, finite element analysis was used to calculate interfacial toughness, Gc, corresponding to the pop-in load. The visualization of crack propagation after pop-in load suggested that the crack had propagated along the interface in all four coatings. The widely accepted tensile adhesion test (ASTM C633) was separately performed on the same four coatings to compare the results obtained from the offset bending test. A correlation between interfacial toughness measured using offset bending and adhesion strength measured using the tensile adhesion test has been proposed.

Investigation of the signal uncertainty in laser-induced breakdown spectroscopy based on error propagation considering self-absorption

The relatively high signal uncertainty of laser-induced breakdown spectroscopy (LIBS) is a key obstacle to its analytical performance and large-scale application. Despite the extensive research efforts to reduce signal uncertainty, few fundamental studies have been conducted because of the complexity of the microscopic mechanisms involved in plasma evolution. Recently, error propagation analysis has provided an inspiring perspective by linking signal uncertainty with plasma property fluctuations, while further investigation is urgently needed due to the limitations of the employed plasma diagnostic techniques. In this study, a spectrum fitting model was proposed for optically thick and homogeneous plasmas to provide more reliable plasma properties for error propagation analysis for the first time. The model was applied to laser-induced copper plasmas using a laser wavelength of 1064 nm and a pulse duration of 25 ns. The uncertainty of an atomic copper line was analyzed at different delay times. Our findings revealed that the fluctuation of columnar total number density was the most significant factor under the investigated conditions, and that the correlation term of temperature and columnar total number density made a considerable negative contribution. Furthermore, self-absorption suppressed the magnitudes of major contribution terms for the signal uncertainty of LIBS.

Proton beam energy deposition in fast ignition and production of protons on Shenguang II upgraded device

The proton beam energy deposition and the prodution of proton beams in proton fast ignition are investigated with the fluid program, partice-in-cell program and Fokker-Planck program based on the parameters of Shenguang II upgraded device. Firstly, according to the target parameters of fast ignition, the energy depositions of different energy protons are investigated. It is obtained that the higher the incident proton energy, the higher the surface density that the protons go through, accordingly the longer the proton deposition distance in the same background plasma density. On the assumption that the diameter of the compression core is 20–30 μm, and that the protons deposited in the core give the energy to the background plasma, the energy of the proton required by fast ignition is obtained by Fokker-Planck simulation. Protons with energy of 7–12 MeV are appropriate for ignition when the background plasma density is 300 g/cm<sup>3</sup>, while 8–18 MeV protons for 400 g/cm<sup>3</sup>. The background plasma temperatures are both 5 keV in the two cases. Secondly, we use particle-in-cell program to study the proton acceleration with or without preplasma which is given by fluid program with using the laser intensity <inline-formula><tex-math id="M1">\begin{document}$ I = 5.4 \times {10^{19}}{\text{ }}{\rm{W/c}}{{\rm{m}}^2} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20222005_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20222005_M1.png"/></alternatives></inline-formula> based on the parameters of Shenguang II upgraded device. The laser has 350 J of enegy, 3 ps of Gaussion pluse width and 10 µm of spot radius. The curvature of the target which is 10 µm thick copper coated with 1 µm thick hydrogen plasma, is 500 µm. The maximum proton energy obtained with preplama is 22 MeV, however the maximum proton energy obtained without preplasma is 17.5 MeV. The conversion efficiency from laser to protons is 5.12% with preplasma and 4.15% without preplasma. The conversion efficiency with preplasma is 20% higher than that without preplasma. We also study the mechanisms of the acceleration in the two situations. The freely expanding plasma model is used to explain the acceleration mechanism. The simulated electric field is smaller than that calculated by using the freely expanding plasma model, because some protons are accelerated at the time of plasma expansion, which consumes some electric field. The results of proton energy deposition show that the proton beams that are suitable for fast ignition can be obtained by the Shenguang II upgraded device.

The relationship between reduced choroidal thickness due to high plasma asymmetrical dimethylarginine level and increased severity of diabetic retinopathy.

To evaluate the relationship between subfoveal choroidal thickness and plasma asymmetrical dimethylarginine level and the severity of diabetic retinopathy in patients with type 2 diabetes mellitus. A total of 68 cases, including 15 patients without diabetic retinopathy, 17 patients with nonproliferative diabetic retinopathy, 16 patients with type 2 diabetes mellitus and proliferative diabetic retinopathy, and 20 healthy patients (control group), were enrolled in this study. Subfoveal choroidal thickness was measured manually using the enhanced depth imaging optical coherence tomography scanning program, and plasma asymmetrical dimethylarginine level was measured using a commercial micro enzyme-linked immunosorbent assay kit. The subfoveal choroidal thickness values and plasma asymmetrical dimethylarginine levels were significantly different between the four groups (p<0.001 and p<0.001). The subfoveal choroidal thickness values were significantly lower in the proliferative diabetic retinopathy group than in the other three groups (no diabetic retinopathy, nonproliferative diabetic retinopathy, and control groups; p<0.001, p=0.045, and p<0.001, respectively). The plasma asymmetrical dimethylarginine levels were significantly higher in the proliferative diabetic retinopathy group than in the other three groups (p<0.001, p<0.04, and p<0.001, respectively). In addition, a significant negative correlation was also found between plasma asymmetrical dimethylarginine level and subfoveal choroidal thickness (p<0.001, r=-0.479). Asymmetrical dimethylarginine is an important marker of endothelial dysfunction and endogenous endothelial nitric oxide synthase inhibitor. The severity of diabetic retinopathy was related to increased plasma asymmetrical dimethylarginine level and reduced subfoveal choroidal thickness in type 2 diabetic patients with diabetic retinopathy.

Electron temperature fluctuation measurements with Correlation Electron Cyclotron Emission in L-mode and I-mode plasmas at ASDEX Upgrade

The Correlation Electron Cyclotron Emission (CECE) diagnostic at ASDEX Upgrade (AUG) is used to investigate the features of outer core and pedestal (ρpol = 0.85-1.0) turbulence across confinement regime transitions. The I-mode confinement regime is a promising operational scenario for future fusion reactors because it features high energy confinement without high particle confinement, but the nature of the edge and pedestal turbulence in I-mode plasmas is still under investigation. The edge Weakly Coherent Mode (WCM) appears in the I-mode pedestal and may play a role in transport. In this work we explore electron temperature (Te) fluctuations in the plasma outer core and pedestal using a 24-channel high radial resolution CECE radiometer. CECE measurements provide turbulence information including the Te fluctuation amplitude, turbulent spectra, and radial localization of turbulent features. With CECE measurements we show that the WCM is localized in the pedestal region in both L-mode and I-mode and is measured in optically thick plasmas with a Te fluctuation amplitude of 2.3%. Broadband drift wave turbulence is measured in the outer core with a Te fluctuation amplitude of &lt;1%. A second CECE system recently installed at AUG allowed for non-standard fluctuation measurements during L-mode and I-mode experiments. The second CECE system was toroidally separated from the primary system, allowing measurements of the long-range toroidal correlation of the WCM indicating its low toroidal mode number. A reflectometer sharing a line of sight with the second CECE system enabled density-temperature cross-phase (αne Te ) measurements. The WCM αne Te changes between L-mode and I-mode as the Te gradient steepens.

Bursts from High-magnetic-field Pulsars Swift J1818.0-1607 and PSR J1846.4-0258

The detection of magnetar-like bursts from highly magnetic (B > 1013 G) rotation-powered pulsars (RPPs) opened the magnetar population to yet another group of neutron stars. At the same time the question arose as to whether magnetar-like bursts from high-B RPPs have similar characteristics to bursts from known magnetar sources. We present here our analyses of the Fermi Gamma-ray Burst Monitor (GBM) data from two magnetar candidates, Swift J1818.0−1607 (a radio-loud magnetar) and PSR J1846.4−0258. Both sources entered active bursting episodes in 2020 triggering Fermi-GBM in 2020 and in early 2021. We searched for untriggered bursts from both sources and performed temporal and spectral analyses on all events. Here, we present the results of our comprehensive burst search and analyses. We identified 37 and 58 bursts that likely originated from Swift J1818.0−1607 and PSR J1846.4−0258, respectively. We find that the bursts from these sources are shorter on average than typical magnetar bursts. In addition, their spectra are best described with a single blackbody function with kT ∼ 10–11 keV; several relatively bright events, however, show higher energy emission that could be modeled with a cutoff power-law model. We find that the correlation between the blackbody emitting area and the spectral temperature for the burst ensemble of each pulsar deviates from the ideal Stefan–Boltzmann law, as it does for some burst-active magnetars. We interpret this characteristic as being due to the significant radiation anisotropy expected from optically thick plasmas in very strong magnetic fields.

Development of miniaturized SAF-LIBS with high repetition rate acousto-optic gating for quantitative analysis

The self-absorption effect in laser-induced breakdown spectroscopy (LIBS) reduces the accuracy of quantitative measurement results. The self-absorption-free LIBS (SAF-LIBS) has been proved to directly capture the optically thin plasma spectra by setting an appropriate exposure time. In this work, a novel SAF-LIBS technique with high repetition rate acousto-optic gating is developed, in which an acousto-optic modulator is used as the shutter to diffract the optically thin fluorescence, and a high repetition rate laser is used to produce quasi-continuous plasmas to enhance the integral spectral intensity, so that the CCD spectrometer can replace an intensified CCD (ICCD) and echelle spectrometer in SAF-LIBS. Experimental results show that the average absolute prediction error of aluminum is reduced to 0.18%, which is equivalent to that of traditional SAF-LIBS. This technique not only effectively shields continuous background radiation and broadened spectral lines in optically thick plasma, but also has advantages of miniaturization, low cost, convenience and reliability.

Ultrastructure of cells constituting lymphoid tubules and circulating hemocytes in Penaeus monodon

The two main groups of cells in the lymphoid tubule wall of Penaeus monodon are fixed cells and migrating hemocytes. Fixed cells include endothelial, stromal, and capsular cells. Together, they form the scaffold that defines the structure of the lymphoid tubule and provide physical support as well as a niche for transmigrating hemocytes. The luminal surface of lymphoid tubule was lined by elongated, spindle-shaped endothelial cells with a centrally located nucleus and rather thick plasma membrane. Stromal cells were the smallest type of fixed cell. They are stellate cells located between the inner endothelial and outer capsular cells. These cells formed a cyto-reticular network for migrating hemocytes. Capsular cells have a flattened and irregular shape with a ruffled border with long filamentous microvilli. The nucleus is centrally located within a small mass of cytoplasm. Together they form the outermost layer of the lymphoid tubular wall. Transmigrating hemocytes within the lymphoid tubules, as opposed to circulating hemocytes, were classified into hyaline (HH), small granular (SGH) and large granular (LGH) hemocytes. The HH have very few granules and a few cytoplasmic organelles, reflecting low synthetic activity. The granular hemocytes (SGH and LGH), despite being different in size, have similar ultrastructural characteristics. They contain high amounts of rough endoplasmic reticulum, ribosomes, mitochondria, and three types of granules. These characteristics implicate their higher synthetic as well as immunologic activities. Based on these characteristics we believe that all the hemocytes belong to a single line of cell differentiation.

ECEI characterization of pedestal fluctuations in quiescent H-mode plasmas in DIII-D

Electron cyclotron emission imaging (ECEI) is employed to characterize the magneto hydraulics dynamics (MHD) fluctuations at the quiescent H-mode pedestals in DIII-D. Pedestal MHD fluctuations cause ECE radiation temperature fluctuations in both the pedestal and scrape-off-layer (SOL). A synthetic ECE platform is utilized for detailed interpretation of the ECEI signals in the SOL and pedestal regions. It is observed that the ECE radiation , which is located in the SOL region according to the cold and optically thick plasma resonance assumption, is extremely sensitive to MHD radial displacements near the separatrix, exhibiting radiation inversion to at the pedestal. Here, the radiation inversion refers to the opposite phase between the radiation fluctuation at the pedestal and the radiation fluctuation at the SOL. Consequently, the quasi-coherent MHD (QCM), which displays a radiation inversion, is found to be consistent with an MHD radial structure that has a strong displacement near the separatrix. In contrast, the edge harmonic oscillation (EHO), which displays weak or no inversion, is found to be consistent with an MHD radial displacement structure peaking at the pedestal top. The ECEI data, interpreted with synthetic ECE, are in qualitative agreement with beam emission spectroscopy measurements on DIII-D for the relative radial extent and localization of the EHO and QCM. The high sensitivity of ECE radiation to separatrix displacements can be used to detect turbulence or MHD fluctuations near the separatrix, which may affect the transport across the separatrix and the wetted area in the divertor. The inversion measured with an ECE or ECEI system potentially provides important information on the magnetic field at the separatrix, which helps constrain the pedestal equilibrium reconstruction and achieve an unambiguous mapping of the ECE/ECEI system with respect to the separatrix.

Positron generation via ultra-intense circularly polarized laser pulses colliding in near-critical-density plasmas with different thickness

The pulses collision scheme has been demonstrated to be an effective way for the Breit–Wheeler positron generation. In this work, positron generation via two circularly polarized laser pulses colliding in near-critical-density plasmas with different thicknesses is further studied. The results show that high flux and high density positrons are generated and collected in 14 μm thick plasmas as extra backflow electrons contribute to radiation. Though the positron yield in 4 μm thick plasmas is lower than that in the thick plasmas, partial positrons are accelerated out of the collision region with a small divergence and quasi-monoenergetic energy. It is found that the Lorentz field dominates positron acceleration and collection. This investigation will further facilitate quality positron generation and application.

Efficient generation of -radiation in a beam-driven thick plasma with oblique density modulation

Recent experiments on the collective interaction of gigawatt electron beams with plasma at the GOL-PET facility have shown that the power of the electromagnetic radiation at the plasma frequency increases tens of times if the starting plasma is created with quasiperiodic radial density gradients. One of the mechanisms capable of providing highly efficient conversion of an unstable beam-driven wave into an electromagnetic one in the presence of periodic density perturbations is the mechanism of plasma antenna. However, earlier this mechanism was considered only for strictly longitudinal modulation of the plasma density, when its efficiency dropped drastically if the transverse size of the plasma significantly exceeded the radiation wavelength. In this work, based on both analytical theory and particle-in-cell modelling, we will show that the presence of oblique modulation of the plasma density makes the antenna mechanism effective even in a thick plasma, if the long-wavelength satellite of the most unstable beam-driven wave falls into resonance with natural plasma oscillations.

Optimization of Mechanical Properties and Thermal Shock Resistance of Lapo4/8ysz Thick Composite Coatings Via Plasma Spraying

Optimization of Mechanical Properties and Thermal Shock Resistance of Lapo4/8ysz Thick Composite Coatings Via Plasma Spraying

Feasibility of a Plasma-Based Intelligent Reflective Surface

Gaseous Plasma Antennas are devices in which an ionized gas (i.e., plasma) is exploited to transmit and receive Electro-Magnetic waves. Their main advantage over metallic systems is the possibility to reconfigure the antenna performance (e.g., radiation pattern) by electronically varying the plasma parameters (e.g., density). Recently, Intelligent Reflecting Surfaces (IRSs) have been proposed to control the environment between transmitting and receiving antennas manipulating the signals reflected. In this work, the feasibility of a plasma-based IRS is investigated. A theoretical model has been developed to assess the use of plasma as a reflecting medium. Numerical simulations have been performed to preliminary design plasma-based IRSs. Two designs of IRSs, relying on plasma properties consistent with the technology at the state-of-the-art, are proposed. The former enables beam steering operations depending on the continuous control of the phase of the reflected wave. The latter exploits a 1-Bit coding strategy to produce specific diffraction patterns. The main advantage of a plasma-based IRS with respect to the metallic counterpart is the possibility to control the phase of the reflected wave, maintaining the magnitude of the reflection coefficient close to the unit. The main drawback of plasma-based systems is the necessity of using thick plasma elements (in the order of the wavelength in the air) to control the phase of the reflected wave over 360 deg. This constraint can be relaxed if digital plasma elements are adopted.

Моделирование генерации гамма-излучения при взаимодействии сильноточных пучков ультрарелятивистских частиц с плазмой

The generation of gamma photons in the interaction of high-current beams of ultrarelativistic particles with thick plasma targets is considered. Efficiency of the beam energy conversion into gamma radiation depending on the target thickness and density is obtained via full-size 3D particle-in-cell simulation. An analytical estimate obtained within the framework of an approximate model is presented, which is in good agreement with the results of numerical simulation. The considered interaction configuration can be a fairly simple and effective way to obtain high-quality gamma beams. Key words